The overall objectives of this project are to study the structure and assembly of leukoviruses, specifically of Rous Sarcoma virus. Comparative studies on the structure of an intermediate in the assembly of picornaviruses and on the relationship of enteroviruses to rhinovirus structure are also being completed as part of the transition of emphasis in this project from picornaviruses to leukoviruses. Our goals for the current year are to (1) determine the origin (host or viral genome) and the virion-associated polypeptides of Rous Sarcoma virus; (2) to elucidate the topographical location and possible function of Rous Sarcoma virus polypeptides other than the glycoproteins (which appear to be envelope proteins), and of the nucleocapsid polypeptide g, (M.W. 14,000 daltons); and (3) to complete studies on the structure of the 15S subunit believed to be an intermediate in the assembly of picornaviruses.